1. Field
The present embodiments relate generally to communications, and more specifically, to radio frequency receivers in communications.
2. Related Art
Radio frequency (RF) receivers used in modern communication systems may support multiple modes and networks, such as 3G wideband code division multiple access (WCDMA) and 2G Global System for Mobile communications (GSM), using time division multiple access (TDMA). A received RF signal in a certain mode may be translated into a specified frequency band and processed to retrieve the information contained in the RF signal. For example, an RF receiver may amplify, filter, and mix an analog received RF signal into in-phase (I) and quadrature (Q) signals that may be converted into digital signals for further processing.
The multiple modes may utilize the same RF receiver architecture but have different frequency ranges and linearity, noise figure, and sensitivity requirements. Noise figure is a measure of degradation of a signal-to-noise ratio caused by components in the RF receiver. A WCDMA system has full duplex functionality where separate receive and transmit signals may be active simultaneously. A higher power transmitted signal may leak into the receive signal in such a system. To relax linearity and noise requirements due to the transmit signal leakage, a conventional WCDMA system may include a surface acoustic wave (SAW) filter prior to a mixer and downstream processing stages. In addition, a conventional WCMDA system may include a low noise amplifier (LNA) to meet sensitivity, noise figure, and gain control requirements. Using a SAW filter, LNA, amplifiers, and other components may result in increased die size, current drain, cost, and complexity, and reduced performance of an RF receiver.
When a RF receiver receives a two-tone continuous wave or amplitude modulation modulated signal, second order intermodulation distortion (IMD2) may be generated. As IMD2 increases, the signal-to-noise ratio (SNR) and second order input intercept point (IIP2) may decrease, resulting in degraded performance of the RF receiver. In a conventional RF receiver, separate I and Q mixers may combine a received RF signal with I and Q pulses generated from a local oscillator. The received RF signal may be connected to the I and Q mixers from a common node, where the I and Q channels may interact because the I and Q mixers are not electrically isolated. This interaction may allow a DC offset and imbalance on one channel to amplify the DC offset and imbalance on the other channel, resulting in an increased overall DC offset and imbalance and degraded IIP2.
When the received RF signal is connected to the I and Q mixers from a common node, calibration of the IIP2 levels of the I and Q channels to optimal levels may be difficult. Because the mismatch and imbalance in each channel may be different, optimizing one channel for optimal IIP2 level may adversely affect the IIP2 level of the other channel, and vice versa, resulting in overall sub-optimal performance. The combined mismatches and imbalances of the channels may also fall outside of the range of calibration. IIP2 calibration may therefore be limited when attempting to optimize both I and Q channels in a conventional RF receiver topology. In addition, adding additional stages to achieve better isolation can increase die area and current, and create additional gain in front of the mixer, which can degrade the input referred IIP2. An additional stage can also create a voltage mode combining point, create additional local oscillator feed thorough paths, and add another block for potential mismatch.